This invention relates to methods and apparatus for treating cross-linked non-plastic rubber, for example vulcanised rubber.
It is known to recover cross-linked (for example vulcanised) non-plastic rubber for re-use. In most of the known methods for re-use of vulcanised rubber, vulcanised rubber scrap material is prepared in one of two forms. Either the treatment may be essentially mechanical, the vulcanised rubber being separated from contaminants such as pieces of metal or textile residues and subsequently ground and graded to provide a free-flowing rubber crumb in which the rubber is still vulcanised and non-plastic for use as the rubber crumb, or the rubber crumb may be further treated by a combination of mechanical work, action of chemical additives and application of heat to form a material which is commonly referred to as "reclaim rubber". "Reclaim rubber" is no longer vulcanised and is once more plastic and capable of shaping and subsequent re-vulcanisation. "Reclaim rubber" takes the form of a homogeneous mass which may be handled in the same way as ordinary unvulcanised rubber.
It will be appreciated that it is considerably more expensive to prepare "reclaim rubber" than it is to prepare a vulcanised rubber crumb from scrap rubber materials.
Vulcanised rubber crumb has been included in unvulcanised rubber compound, usually in minor proportions, to improve ease of processing during shaping, such as extrusion or calendering prior to vulcanisation. Of increasing importance has been the inclusion of vulcanised rubber crumb to reduce the cost of the final rubber product since scrap rubber is generally available at lower cost than unvulcanised rubber compound. However, there is a disadvantage that the inclusion of vulcanised rubber crumb in unvulcanised rubber compounds results in weaker products being obtained when the rubber compounds are vulcanised. Weakening becomes particularly apparent when vulcanised rubber crumb is added to the unvulcanised rubber compound in an amount of the order of 25% by weight or more in terms of the final product.
These mechanical weaknesses are believed to be due to the occurrence on deformation of concentrations of stress at the interfaces between the particles of the vulcanised rubber crumb and the rubber compounds in which they are incorporated. These interfaces are present because the surfaces of the vulcanised rubber crumb are non-plastic and are, therefore, not miscible with the surrounding rubber compound before that rubber compound is vulcanised to form the final product. In consequence a clearly defined multiplicity of adhesive interfaces is present and these give rise to the stress concentrations which lead to mechanical weakness. Accordingly the re-use of vulcanised rubber crumb with rubber compounds which are subsequently vulcanised seldom yields a product which is comparable to the same article made from a newly vulcanised rubber compound.
On the other hand the fully plasticised "reclaim rubber" which is obtained by breaking down the vulcanisation of the rubber crumb suffers from the disadvantage that it is so weakened by the reclaiming process that products obtained on subsequent revulcanisation exhibit only relatively moderate strength which is significantly less than the strength of the vulcanised rubber from which the "reclaim rubber" is prepared.